Long wavelength vertical cavity surface emitting laser

ABSTRACT

A VCSEL for emitting long wavelength light including a GaAs substrate element, a first mirror stack with mirror pairs in a GaAs/AlGaAs material system lattice matched to a GaInAsN active region. The active region including an active structure sandwiched between a first cladding region adjacent the first mirror stack, and a second cladding region. The first and second cladding regions including an AlGaAs/GaAs material system. The active structure including a nitride based quantum well and a GaAsP barrier layer. A second mirror stack is lattice matched to the second cladding region and has mirror pairs in a GaAs/AlGaAs material system.

FIELD OF THE INVENTION

This invention relates to vertical cavity surface emitting lasers andmore specifically to vertical cavity surface emitting lasers foremitting long wavelength light.

BACKGROUND OF THE INVENTION

Vertical cavity surface emitting lasers (VCSEL) include a firstdistributed Bragg reflector (DBR), also referred to as a mirror stack,formed on top of a substrate by semiconductor manufacturing techniques,an active region formed on top of the first mirror stack, and a secondmirror stack formed on top of the active region. The VCSEL is driven bya current forced through the active region, typically achieved byproviding a first contact on the reverse side of the substrate and asecond contact on top of the second mirror stack.

The use of mirror stacks in VCSELs is well established in the art.Typically, mirror stacks are formed of multiple pairs of layers oftenreferred to as mirror pairs. The pairs of layers are formed of amaterial system generally consisting of two materials having differentindices of refraction and being easily lattice matched to the otherportions of the VCSEL. For example, a GaAs based VCSEL typically uses anAlAs/GaAs or AlAs/AlGaAs material system wherein the differentrefractive index of each layer of a pair is achieved by altering thealuminum content in the layers. In conventional devices, the number ofmirror pairs per stack may range from 20-40 to achieve a high percentageof reflectivity, depending on the difference between the refractiveindices of the layers. The large number of pairs increases thepercentage of reflected light.

In conventional VCSELS, conventional material systems performadequately. However, new products are being developed requiring VCSELswhich emit light having long wavelengths. VCSELs emitting light having along wavelength are of great interest in the optical telecommunicationindustry. As an example, a long wavelength VCSEL can be obtained byusing a VCSEL having an InGaAs/InGaAsP active region. When anInGaAs/InGaAsP active region is used, an InP/InGaAsP material systemlattice matched to the InP substrate must be used for the mirror stacksin order to achieve a lattice match. In this system, however, it ispractically impossible to achieve decent DBR based mirrors because ofthe insignificant difference in the refractive indices in this materialsystem. Many attempts have been made to address this problem including awafer bonding technique in which a DBR mirror is grown on a separatesubstrate and bonded to the active region. This technique has had onlylimited success and also the interface defects density in the waferfusion procedure causes potential reliability problems.

It would be highly advantageous, therefore, to remedy the foregoing andother deficiencies inherent in the prior art.

Accordingly, it is an object of the present invention to provide a newand improved long wavelength VCSEL.

Another object of the invention is to provide a reliable, long life longwavelength VCSEL.

Still another object of the immediate invention is to provide anefficient active region and mirror stacks for use in a long wavelengthVCSEL.

Yet another object of the invention is to reduce the complexity offabricating a long wavelength VCSEL.

Another object of the present invention is to provide an active regionwhich emits long wavelength light and a mirror stack which can belattice matched thereto.

SUMMARY OF THE INVENTION

Briefly, to achieve the desired objects of the instant invention inaccordance with a preferred embodiment thereof, provided is a VCSEL foremitting long wavelength light. The VCSEL includes a GaAs substrateelement, a first mirror stack disposed on the substrate element, aGaInAsN/GaAsP active region, including at least one GaInAsN basedquantum well sandwiched between a plurality of GaAsP barrier layers, anda plurality of AlGaAs/Gas cladding layers. The active region is disposedon the first mirror stack, and a second mirror stack is disposed on theactive region.

In a preferred embodiment the active region and the first and the secondmirror stacks are configured to emit light with a wavelength in a rangeof approximately 1.3-1.55 micrometers. The quantum well is configuredwith a direct energy band-gap in a range of approximately 1.0 to 0.8 eV.

Also provided is a method of fabricating a VCSEL for emitting longwavelength light. The method includes providing a GaAs substrate havinga surface, epitaxially growing a first mirror stack on the surface,epitaxially growing an AlGaAs/GaAs cladding region on the first mirrorstack, epitaxially growing a GaInAsN/GaAsP active structure including atleast one quantum well on the first cladding region, epitaxially growinga second GaAs/AlGaAs cladding region on an upper surface of the activestructure and epitaxially growing a second mirror stack on the secondcladding region.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further and more specific objects and advantages ofthe instant invention will become readily apparent to those skilled inthe art from the following detailed description of a preferredembodiment thereof taken in conjunction with the drawings, in which:

FIG. 1 is a sectional view of a VCSEL in accordance with the presentinvention; and

FIG. 2 is a sectional view of the active region of the VCSEL of FIG. 1in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawings in which like reference characters indicatecorresponding elements throughout the several views, attention is firstdirected to FIG. 1 which illustrates a long wavelength vertical cavitysurface emitting laser (VCSEL) generally designated 10. Typically,VCSELs are formed by depositing a plurality of layers on a substrate toform the VCSEL. See, for example, U.S. Pat. No. 5,034,092, entitled"PLASMA ETCHING OF SEMICONDUCTOR SUBSTRATES", issued Jul. 23, 1991,assigned to the same assignee and included herein by this reference, andU.S. Pat. No. 5,256,596, entitled "TOP EMITTING VCSEL WITH IMPLANT",issued Oct. 26, 1993, assigned to the same assignee and included hereinby this reference. VCSEL 10 of the present invention is formed on asubstrate 12, which in this specific embodiment, is GaAs, morespecifically either GaAs (100) or GaAs (111). GaAs is used to facilitateepitaxial growth of the components of VCSEL 10 which emits light in longwavelengths approximately in the range of 1.3 μm-1.55 μm. While itshould be understood that either GaAs (100) or GaAs (111) can be usedfor substrate 12, when GaAs (111) is employed, the surface crystalorientation will allow for longer wavelength continuous wave (CW)operation at normal operating temperatures. More specifically, the useof a GaAs material having a (111) oriented substrate will allow for theextension of an emitted wavelength up to 1.1 μm when only InGaAs is usedin the active region. This wavelength extension is extremely hard toachieve on a (100) substrate surface crystal orientation.

Substrate 12 has an upper surface 13 on which a mirror stack 14 isdisposed. Mirror stack 14 includes a plurality of mirror pairs in aGaAs/AlGaAs material system. An active region 20 is disposed on mirrorstack 14. Active region 20, as further detailed in FIG. 2, includes anactive structure 23 sandwiched between a first cladding region 24adjacent first mirror stack 14 and a second cladding region 25. A secondmirror stack 26 is disposed on second cladding region 25 and includesmirror pairs in a GaAs/AlGaAs material system.

Mirror stack 14 is grown by epitaxially depositing pairs of layers onsubstrate 12. In order to crystal lattice match mirror stack 14 tosubstrate 12 a suitable semiconductor material system must be deposited.In this specific example, substrate 12 is GaAs and, therefore, aGaAs/AlGaAs material system is employed. Approximately 20-40 mirrorpairs of this material system are deposited on substrate 12 depending onthe difference between the refractive indices of the layers. Thedifferent refractive index the layers of each pair is achieved byaltering the aluminum content. In this specific embodiment, aGaAl.sub..7 As layer 15 and a GaAs layer 16 forming a mirror pair ispreferred. The large number of pairs increases the percentage ofreflected light.

Referring again to FIGS. 1 and 2, cladding region 24 includes one ormore layers which may be graded if necessary for more efficient carrierand optical confinement in active structure 23. In this specificembodiment, cladding region 24 is formed of a AlGaAS/GaAs materialsystem. This combination of materials allows for increased carrier andoptical confinement in VCSEL 10. For example cladding region 24 includesa first layer 30 formed of a n-type AlGaAs whereby the aluminum molefraction ranges from 20-30% to assure a better confinement, and a secondlayer 31 formed of a material having a gradient to efficiently confinethe carriers in active structure 23. In this particular embodiment,first layer 30 is n-doped AlGaAs using silicon, or any other similardopant, and second layer 31 is formed of undoped GaAs.

Active structure 23, in this embodiment, includes three nitride basedquantum well layers 35, 36, and 37, separated by barrier layers 38 and39. For example quantum well layers 35, 36, and 37 and barrier layers 38and 39 are each approximately 100Å and the total thickness of activeregion 20 is approximately one wavelength of the emitted light or amultiple thereof. Quantum well layers 35, 36, and 37 are formed ofGa_(1-y) In_(y) As_(1-x) (N)_(x), more specifically Ga₀.75 In₀.25As_(1-x) N_(x), where x=0.0001-0.1, and barrier layers 38 and 39 areformed of GaAsP for strain compensation. More specifically, the GaInAsNquantum wells are under compression, and the GaAsP barrier layers areunder tension. Therefore, due to this strain compensation, the indiumcomposition in the quantum well layers 35, 36, and 37 can be increasedin a range of 5-10%. This increase in the indium material allows for alonger wavelength emission without the generation of misfit dislocationsutilizing the same thickness of active layer (100Å). In addition, itshould be understood that dependent upon the crystal orientation of theGaps substrate 12, a higher indium composition can be utilized. Oneskilled in the art will understand that more or fewer quantum welllayers and barrier layers can be used to compose active structure 23depending upon the application. Active region 20 and first and secondmirror stacks 14 and 26 respectively are configured to emit light with awavelength in a range of approximately 1.3-1.55 micrometers. To achievethis range the quantum wells are configured with a direct energyband-gap in a range of approximately 1.0 eV to 0.8 eV. The mole fractionof the indium is higher than that found in a typical VCSEL structure dueto the use of the GaAsP barrier layer. In that the incorporation of anitride based quantum well active region is difficult at low growthtemperatures, the use of a GaAs (111) substrate would further facilitatethe achievement of a longer wavelength emission by adding a smallfraction of nitrogen to the InGaAs. This fraction is lower than the onerequired for a GaAs (100) substrate.

Cladding region 25 includes one or more layers which may be graded ifnecessary for more efficient carrier and optical confinement in activestructure 23. In this specific embodiment, cladding region 25 is formedof an AlGaAs/GaAs material system. For example cladding region 25includes a first layer 40 formed of p-type AlGaAs to lattice matchmirror stack 26. A second layer 41 is formed of a material having agradient to more efficiently confine carriers in active structure 23. Inthis particular embodiment, second layer 41 is formed of undoped GaAs.Again, the aluminum mole fraction in layer 40 of AlGaAs ranges fromabout 20-30% to assure a better confinement and is p-doped utilizingcarbon, or any other similar dopant.

Mirror stack 26 is grown by epitaxially depositing pairs of layers oncladding region 25. In order to crystal lattice match mirror stack 26and active structure 23, a suitable semiconductor material system mustbe deposited. In this specific example, cladding region 25 isAlGaAs/GaAs based and, therefore, a GaAs/AlGaAs material system isemployed. Approximately 20-40 mirror pairs of this material system aredeposited on cladding region 25 depending on the difference between therefractive indices of the layers. The different refractive index thelayers of each pair is achieved by altering the aluminum content. Inthis specific embodiment, a GaAl.sub..7 As layer 27 and a GaAs layer 28forming a mirror pair is preferred. The large number of pairs increasesthe percentage of reflected light.

To complete VCSEL 10, a contact layer 45 is positioned on mirror stack26, and a contact layer 46 is positioned on substrate 12, for example onthe rear surface thereof. In this particular embodiment, contact layers45 and 46 are formed of GaAs. As will be understood by those skilled inthe art contact 45 is so constructed as to permit the emission of lightfrom VCSEL 10.

Various changes and modifications to the embodiments herein chosen forpurposes of illustration will readily occur to those skilled in the art.For example, it should be understood that VCSEL structure symmetryexists for both the p and n dopants as well as electrically invertedstructure designs. To the extent that such modifications and variationsdo not depart from the spirit of the invention, they are intended to beincluded within the scope thereof which is assessed only by a fairinterpretation of the following claims.

While we have shown and described specific embodiments of the presentinvention, further modifications and improvements will occur to thoseskilled in the art. We desire it to be understood, therefore, that thisinvention is not limited to the particular forms shown and we intend inthe append claims to cover all modifications that do not depart from thespirit and scope of this invention.

What is claimed is:
 1. A vertical cavity surface emitting laser foremitting long wavelength light, the vertical cavity surface emittinglaser comprising:a GaAs substrate; a first mirror stack disposed on theGaAs substrate; a first cladding region including an AlGaAs/GaAsmaterial system disposed on the first mirror stack; a GaInAsN activestructure including a nitride based quantum well and a barrier layer,the active structure disposed on the first cladding region; a secondcladding region including an AlGaAs/GaAs material system disposed on theactive structure; and a second mirror stack disposed on the secondcladding region, wherein the first and second cladding regions, and thefirst and the second mirror stacks are configured to emit light with awavelength in a range of approximately 1.3-1.55 micrometers.
 2. Avertical cavity surface emitting laser as claimed in claim 1 wherein thebarrier layer is GaAsP for strain compensation.
 3. A vertical cavitysurface emitting laser as claimed in claim 1 wherein the first claddingregion and the second cladding region include an aluminum mole fractionin a range of 20-30%, thereby achieving better confinement.
 4. Avertical cavity surface emitting laser as claimed in claim 3 wherein thefirst and second cladding regions are doped with one of silicon andcarbon, thereby characterized as being one of n-doped and p-doped,respectively.
 5. A vertical cavity surface emitting laser as claimed inclaim 1 wherein the nitride based quantum well is configured with adirect energy band-gap in a range of approximately 1.0 to 0.8 eV.
 6. Avertical cavity surface emitting laser as claimed in claim 1 wherein thenitride based quantum well includes Ga_(1-y) In_(y) As_(1-x) (N)_(x)wherein x has a range of 0.001-0.1.
 7. A vertical cavity surfaceemitting laser as claimed in claim 1 wherein the nitride based quantumwell includes Ga₀.75 In₀.25 As_(1-x) N_(1-x) wherein x has a range of0.001-0.1.
 8. A vertical cavity surface emitting laser for emitting longwavelength light, the vertical cavity surface emitting lasercomprising:a GaAs substrate; a first mirror stack including mirror pairsin a GaAs/AlGaAs material system adjacent the GaAs substrate; a GaInAsNactive region including an active structure sandwiched between a firstAlGaAs/GaAs cladding region adjacent the first mirror stack and a secondAlGaAs/GaAs cladding region, the active structure including a nitridebased quantum well and a GaAsP barrier layer; and a second mirror stackdisposed on the second AlGaAs/GaAs cladding region and including mirrorpairs in a GaAs/AlGaAs material system, wherein the active region, andthe first and the second mirror stacks are configured to emit light witha wavelength in a range of approximately 1.3-1.55 micrometers.
 9. Avertical cavity surface emitting laser as claimed in claim 8 wherein thefirst cladding region and the second cladding region include an aluminummole fraction in a range of 20-30%, thereby achieving betterconfinement.
 10. A vertical cavity surface emitting laser as claimed inclaim 9 wherein the first and second cladding regions are graded formore efficient carrier confinement and optical confinement in the activestructure.
 11. A vertical cavity surface emitting laser as claimed inclaim 8 wherein the nitride based quantum well is configured with adirect energy band-gap in a range of approximately 1.0 to 0.8 eV.
 12. Avertical cavity surface emitting laser as claimed in claim 11 whereinthe nitride based quantum well includes Ga_(1-y) In_(y) As_(1-x) (N)_(x)wherein x has a range of 0.001-0.1.
 13. A vertical cavity surfaceemitting laser as claimed in claim 12 wherein the nitride based quantumwell includes Ga₀.75 In₀.25 As_(1-x) N_(x) wherein x has a range of0.001-0.1.